Lipid A modification is associated with colistin susceptibility in Serratia nevei clinical isolates

Serratia spp. are recognised for intrinsic resistance to colistin, attributed to modifications of the lipid A moiety of lipopolysaccharide (LPS). We characterised 107 Serratia isolates from bacteraemia (2005-2020), analysed natural mutations in LPS-related genes, and examined interactions with murine macrophages. Eight isolates carried loss-of-function mutations in lpxL1, lpxL2, mgrB, or htrB1. Mutants, particularly those with mgrB disruption, were resistant to colistin despite altered lipid A. Lipid A ions at m/z 2405, 2417, and 2439 were more abundant in mutants, whereas wild-type strains predominantly showed m/z 1347, 1372, and 1390 forms. Of the 63 Serratia nevei studied, four of them (6.3%) were colistin-susceptible (MIC ≤ 1 mg/L) and displayed distinct lipid A profiles, with higher relative abundance of m/z 1842 and 1389 variants compared to the m/z 1347 form enriched in resistant isolates. In vitro assays revealed that colistin-resistant isolates exhibited reduced adherence and increased phagocytosis by macrophages. However, cytokine induction (IL-1β, CXCL10) and macrophage toxicity appeared to be associated with phylogenetic background rather than resistance phenotype. These findings elucidate mechanisms of intrinsic colistin resistance in Serratia, highlight susceptibility in certain Serratia. nevei isolates, and reveal the interplay between lipid A structure, resistance, and host-pathogen interactions. Importance: Serratia spp. are increasingly recognized as challenging nosocomial pathogens due to their broad antibiotic resistance, including intrinsic resistance to colistin. We described that colistin susceptibility in clinical Serratia isolates is more varied than previously assumed and is linked to the natural absence of key lipopolysaccharide (LPS) biogenesis genes. Furthermore, our results demonstrate that LPS alterations, driven by antibiotic resistance mechanisms, concurrently reshape the bacterium's interaction with the host immune system, particularly macrophages. The dual impact of our results has significant implications for understanding Serratia pathogenesis, guiding therapeutic choices, and developing novel strategies against these adaptable pathogens by providing insights into how antibiotic resistance evolution can directly determine virulence and immune evasion.